In a wideband radio access system, a mobile subscriber station (MSS) can enter sleep mode in order to minimize power consumption of the MSS. The MSS in sleep mode follows the operating schedule according to sleep interval which increases at a specified ratio. The sleep interval is comprised of listening window and sleep window.
The listening window is determined according to a sleep request message and a sleep response message. More specifically, the MSS transmits a request to a base station (BS) asking for permission to enter sleep mode. In response to the request, the BS transmits a response message granting the MSS to enter sleep mode.
The MSS enters sleep mode upon receiving the sleep response message from the BS. The MSS remains in traffic, which has its address, during a listening window.
During the listening window period, a traffic indication (MOB-TRF-IND) message is broadcasted from the BS. From the traffic indication message, the MSS checks to determine whether there is any downlink traffic having its address.
During sleep mode, the MSS receives minimum number or amount of downlink signal(s) from the BS in order to save power. In addition, the MSS scans neighboring base stations for performing handover and performs ranging operation to maintain appropriate downlink coding type in order to maintain uplink transmission and signal quality.
The operation of sleep mode includes communication of medium access control (MAC) messages such as sleep request (MOB-SLP-REQ) message, sleep response (MOB-SLP-RSP) message, and traffic indication (MOB-TRF-IND) message(s).
Table 1 illustrates an example of a sleep request message which includes sizes of sleep window and listening window.
TABLE 1SizeSyntax(bits)NotesSLP-REQ Message Format( ) { Management message8type = 50 initial sleep window6 final sleep window10 listening window4 final sleep window3exponent N_sleep_CID8Number of Sleep CID For (i=0;i<N_Sleep_CID; i++{  Sleep_CID16 } reserved1}
Table 2 illustrates an example of a sleep response message which includes information such as whether the request for sleep mode entry has been granted, sizes of sleep window and listening window, and sleep identification.
TABLE 2SizeSyntax(bits)NotesMOB-SLP-RSP MessageFormat( ) { Management message8type = 51 Sleep-approved10: Sleep Moderequest denied1: Sleep Moderequest approved If (Sleep-approved = 0){  REQ-duration7Time duration } else {  If (SHO or FBSSIf SHO or FBSScapability enabled) {capability isenabled in the REG-REQ-RSP messageexchange   Maintain11: Active set andActive Set and AnchorAnchor BS ID isBS IDmaintained while insleep mode forSHO/FBSS duration0: Active set andAnchor BS ID is notmaintained while insleep mode    If(Active Set andAnchor BS IDmaintained) {    SHO/FBSS3Active set andduration (5)Anchor BS ID ismaintained for10×2{circumflex over ( )}s frames afterentering sleep mode   }  }  Start frame6  initial sleep6window  final sleep window10  listening window4  final sleep3window exponent  SLPID10  Sleep duration8In units of 20 msframes type/length/value (TLV )encoded information } PaddingvariableTo ensure octet-aligned}
As explained above, upon receipt of the sleep response message, the MSS enter sleep mode. An operation of the MSS in sleep mode is explained below. FIG. 1 illustrates an example of operation of the MSS in sleep mode. More specifically, FIG. 1 illustrates the MSS transmitting to the BS a request to enter sleep mode, and terminating sleep mode upon notice from the BS of downlink traffic to the MSS. The details of each operation are as follows.
The MSS includes in the sleep request message (See Table 1) information, such as an initial sleep interval (N1), final sleep interval (N2), and listening interval (L1), in the request to enter sleep mode and transmits sleep mode entry request to the BS (S11). In response, the BS determines the initial sleep interval (N1), final sleep interval (N2), listening interval (L1), and sleep mode start time (M), and transmit the sleep response message (See Table 2) including the determined information to the MSS, assuming the BS approves the sleep mode entry request (S12).
The MSS maintains sleep mode for initial sleep interval (N1) after entering sleep mode. After the initial sleep mode expires, the MSS receives a traffic indication message, which is broadcasted, during the listening interval (L1) (S13). If the MSS ascertains that there is no downlink traffic addressed to it, the MSS increases the sleep interval by two-folds from the initial sleep interval (2*N1).
More specifically, a subsequent sleep interval increases two-folds from the previous sleep interval until the final sleep interval, whereupon the duration of the final sleep window is maintained and repeated. The final sleep window is determined using the parameters of Table 2 and is as follows.Final sleep window=Final sleep window base*2final window exponent  [Equation 1]
During the listening window, if the MSS receives via the traffic indication message that there is traffic addressed to the MSS (S15), the MSS terminates sleep mode and returns to normal operation. After returning to normal operation, the MSS can receive the downlink traffic.
If the MSS returns and remains in sleep mode, the sleep window is twice as long as the previous sleep window. In the subsequent listening window, if there is no downlink traffic addressed to the MSS, and as a result, the MSS returns to sleep mode, again, the duration of sleep interval is twice is long as the previous sleep window. In other words, the sleep window increases at a specified amount, e.g., two times or double, after the MSS returns to sleep mode after each listening window. This is illustrated in FIG. 2.
FIG. 3 is an example of at least two MSSs updating sleep window after entering sleep mode. As illustrated in FIG. 3, if a plurality of MSSs enter sleep mode, there can be numerous listening windows which do not correspond with each other since the MSSs likely enter sleep mode at different times. Consequently, the BS broadcasts the traffic indication messages at every listening window, resulting in increasing number of broadcasts as the number of MSSs increases. Evidently, the problem with this is that the traffic indication message is broadcasted at each and every listening window and with the increase of MSSs, a number of listening windows corresponding to each MSS increases, resulting in increasing number of broadcasts.
In addition, with increasing number of broadcasts, the amount of transmitted data also increases for a MSS. More specifically, the length of the traffic indication message transmitted to a MSS, which is in sleep mode, can become long.